The present invention relates to a primary pressure-balanced proportioning valve which includes a vent controlled bypass, in particular a proportioning valve which closes communication with the vent in order to utilize trapped fluid pressure to terminate proportioning valve operation.
In general, proportioning valves utilize a differential area piston in the secondary chamber brake system so that when a predetermined pressure level is reached the piston shifts in the direction of the smaller area proportioning valve end. As the piston shifts, the piston contacts a poppet or seal and thus establishes a pressure restriction. Upon a further increase in secondary pressure communicated to the proportioning valve piston, the outlet pressure will rise at a rate determined by the differential areas at each end of the piston. The proportioning valve may include a bypass function so that upon failure of pressure in a chamber of the master cylinder, the proportioning valve responds appropriately and permits full fluid pressure communication to the rear brakes of the vehicle. Such existing designs contain numerous disadvantages such as: the primary and secondary pressure chambers of the master cylinder must be reversed which lengthens the master cylinder body; the master cylinder becoming stroke sensitive rather than pressure sensitive; the system experiencing undesirable fluid displacement loss; additional costly mechanical hardware being required in order to effect a bypass function; additional seals being required; and a widening break-point tolerance resulting. It is highly desirable to provide a proportioning valve assembly with a bypass feature wherein it is not necessary to switch the position of the primary and secondary chambers of the master cylinder. It is also desirable to actuate the proportioning valve assembly by communicating directly pressure from the primary pressure chamber of the master cylinder to the proportioning valve assembly. It is also very desirable to effect the bypass function simply by closing off a vent opening so that trapped fluid pressure effects the bypass function.